Contemporary industrial activities generate gaseous effluents containing a multitude of chemical compounds and contaminants which interfere with the equilibrium of elements in nature and affect the environment at different levels. Acid rain, the green-house effect, smog and the deterioration of the ozone layer are examples that speak volumes about this problem. Reduction of noxious emissions is therefore not surprisingly the subject of more and more legislation and regulation. Industrial activities and applications which must contend with stricter environmental regulatory standards in order to expect any long term commercial viability, will turn more and more to biological and environmentally safe methods. Consequently, there is a real need for new apparatus and methods aimed at the biological treatment of gaseous waste or effluents.
There already exists a vast array of technologies aimed at the separation and recovery of individual or mixed gases and a number of different biological methods is known to treat gaseous waste or effluents: bacterial degradation (JP 2000-287679; JP2000-236870), fermentation by anaerobic bacteria (WO 98/00558), photosynthesis through either plants (CA 2,029,101 A1; JP04-190782) or microorganisms (JP 03-216180). Among the more popular are those gained through the harnessing of biological processes such as peat biofilters sprinkled with a flora of microorganisms in an aqueous phase, or biofilter columns comprising immobilized resident microorganisms (Deshusses et al. (1996) Biotechnol. Bioeng. 49, 587-598). Although such biofilters have contributed to technological advances within the field of gaseous waste biopurification, the main drawbacks associated with their use are their difficult maintenance and upkeep, lack of versatility, as well as time consuming bacterial acclimation and response to perturbation periods (Deshusses et al.).
A number of biological sanitation/purification methods and products is known to use enzymatic processes, coupled or not to filtration membranes (S5250305; U.S. Pat. No. 4,033,822; JP 63-129987). However, these are neither intended nor adequate for the cleansing of gaseous waste or effluents. The main reason for this is that, in such systems, contaminants are generally already in solution (U.S. Pat. No. 5,130,237; U.S. Pat. No. 4,033,822; U.S. Pat. No. 4,758,417; U.S. Pat. No. 5,250,305; WO97/19196; JP63-129987). Efficient enzymatic conversion and treatability itself of gaseous waste or effluents in liquids therefore depend on adequate and sufficient dissolution of the gaseous phase in the liquid phase. However, the adequate dissolution of gaseous waste or effluents into liquids for enzymatic conversion poses a real problem which constitutes the first of a series of important limitations which compound the problem of further technological advances in the field of gas biopurification.
Although triphasic <<Gas-Liquid-Solid>> (GLS) reactors are commonly used in a large variety of industrial applications, their utilization remains quite limited in the area of biochemical gas treatment (U.S. Pat. No. 6,245,304; U.S. Pat. No. 4,743,545). Also known in the prior art are the GLS bioprocesses abundantly reported in the literature. A majority of these concerns wastewater treatment (JP09057289). These GLS processes are characterized in that the gaseous intake serves the sole purpose of satisfying the specific metabolic requirements of the particular organism selected for the wastewater treatment process. Such GLS treatment processes are therefore not aimed at reducing gaseous emissions.
As previously mentioned, these systems are neither intended nor adequate for the treatment of gaseous waste or effluents. An additional problem associated with the use of these systems is the non retention of the solid phase within the reactor. Biocatalysts are in fact washed right out of the reactors along with the liquid phase. Different concepts are, nonetheless, based on this principle for the reduction of gaseous emissions, namely carbon dioxide. Certain bioreactors allow the uptake of CO2 by photosynthetic organisms (CA229101; JP03-216180) and similar processes bind CO2 through algae (CA2232707; JP08-116965; JP04-190782; JP04-075537). However, the biocatalyst retention problem remains largely unaddressed and constitutes another serious limitation, along with gaseous effluent dissolution, to further technological advancements.
The main argument against the use of ultrafiltration membranes to solve this biocatalyst retention problem is their propensity to clogging. Clogging renders them unattractive and so their use is rather limited for the retention of catalysts within reactors. However, a photobioreactor for medical applications as an artificial lung (WO9200380; U.S. Pat. No. 5,614,378) and an oxygen recovery system (U.S. Pat. No. 4,602,987; U.S. Pat. No. 4,761,209) are notable exceptions making use of carbonic anhydrase and an ultrafiltration unit.
The patent applications held by the assignee, CO2 Solution Inc., via Les Systèmes Envirobio Inc. (EP0991462; WO9855210; CA2291785) proposes a packed column for the treatment of carbon dioxide using immobilized carbonic anhydrase without the use of an ultrafiltration membrane. Carbonic anhydrase is a readily available and highly reactive enzyme that is used in other systems for the reduction of carbon dioxide emissions (U.S. Pat. No. 4,602,987; U.S. Pat. No. 4,743,545; U.S. Pat. No. 5,614,378; U.S. Pat. No. 6,257,335). In the system described by Trachtenberg for the carbonic anhydrase treatment of gaseous effluents (U.S. Pat. No. 6,143,556; CA2222030), biocatalyst retention occurs through a porous wall or through enzyme immobilization. However, important drawbacks are associated with the use of enzyme immobilization, as will be discussed below.
Other major drawbacks are associated with the use of enzymatic systems. One of these stems from systems where enzymatic activity is specifically and locally concentrated. This is the case with systems where enzymes are immobilized at a particular site or on a specific part of an apparatus. Examples in point of such systems are those where enzymes are immobilized on a filtration membrane (JP60014900008A2; U.S. Pat. No. 4,033,822; U.S. Pat. No. 5,130,237; U.S. Pat. No. 5,250,305; JP54-132291; JP63-129987; JP02-109986; DE3937892) or even, at a gas-liquid phase boundary (WO96/40414; U.S. Pat. No. 6,143,556). The limited surface contact area obtainable between the dissolved gas substrate, the liquid and the enzyme's active site poses an important problem. Hence, these systems generate significantly greater waste of input material, such as expensive purified enzymes, because the contact surface with the gaseous phase is far from optimal and limits productive reaction rates. Therefore, as mentioned previously, overcoming the contact surface area difficulty should yield further technological advances.
Other examples of prior art apparatuses or methods for the treatment of gas or liquid effluent are given in the following documents: CA2160311; CA2238323; CA2259492; CA2268641; JP2000-236870; JP2000-287679; JP2000-202239; U.S. Pat. No. 4,758,417; U.S. Pat. No. 5,593,886; U.S. Pat. No. 5,807,722; U.S. Pat. No. 6,136,577; and U.S. Pat. No. 6,245,304.